The present invention relates to a current measuring system, and more particularly, to a current measuring system for a battery system of an electric vehicle.
Generally, an electric vehicle uses a battery as an energy storage device. In particular, a general electric vehicle uses multiple 12V battery modules connected in series, in order to establish a relatively high voltage. The number of battery modules used is determined by the voltage requirements of the vehicle, which generally averages between 250V and 350V. To obtain these voltage levels using 12V batteries, about 20-30 battery modules must be connected in series.
In electric vehicles, in order to provide enough power to move the vehicle, a battery must provide 200 to 500A of current at the voltage range stated above. To charge the battery, a current of between 10 and 100A is used.
Methods for measuring battery current in general electric vehicles include:
1. a current-measuring method using a shunt resistor;
2. a current-measuring method using an open-loop Hall sensor; and
3. a current-measuring method using a closed-loop Hall sensor.
While the sensors used by the open- and closed-loop Hall sensor methods are different than the sensor of the shunt-resistor method, the overall circuits and methods are similar.
The shunt-resistor method uses a simple and cheap sensor, but its drawbacks include measuring errors and a low stability. This technique is also very sensitive to electrical noise and mechanical interference. Therefore, this technique is not practical for use in electric vehicles.
In contrast, the sensors used in the open- and closed-loop Hall sensor methods are more expensive than the sensor used in the shunt-resistor method, but they are more accurate and have more advantages than the sensor of the shunt-resistor method.
The closed-loop Hall sensor method is commonly used in electric vehicles because its sensor has less noise sensitivity than the open-loop Hall sensor method. However, the closed-loop Hall sensor is very expensive.
In a current-measuring system using a closed-loop Hall sensor, the current measuring system may include an electronic control unit for a battery management system (hereinafter referred to as a BMS ECU), and a sensor for measuring the current (e.g., a current transducer), installed outside the BMS ECU. The characteristics of the external current transducer need to be considered when designing the BMS ECU. Also, when the current sensor is changed, the BMS ECU must also be changed.
In this system, the current transducer utilizes a closed-loop Hall effect current sensor to reduce electrical noise effects. The closed-loop Hall sensor generates a current with a density that is proportional to a density of the measured current. For this reason, the current transducer needs both a positive and a negative power source.
The BMS ECU in this system may include a voltage follower, a reference adjuster, and an analog-to-digital converter (hereinafter referred to as an A/D converter). The voltage follower acts as a buffer in order to cancel out measurement errors caused by impedance changes of the measured circuit. The reference adjuster amplifies an input voltage to a reference voltage when converting an analog signal to a digital signal, because the measured value of the closed loop sensor may be a positive voltage or a negative voltage, according to current direction. The A/D converter performs an analog-to-digital conversion of the measured current.
This type of system has several disadvantages, including:
1. The circuit of the BMS ECU must match the type of the current transducer (i.e., if one is changed, the other must be changed).
2. The BMS ECU can have a complex structure, including a buffer for sensing a current, a voltage distributor, and an A/D converter.
3. The circuitry for the BMS ECU can also be complex because the power for the current transducer is supplied by the BMS ECU.
4. The current transducer must utilize an expensive closed-loop Hall sensor because the system is sensitive to electrical noise.
5. The current-measuring system is not adaptable to other control units of the electrical vehicle.
In a preferred embodiment of the present invention, a system for measuring a current of a battery for an electric vehicle includes a current transducer, a voltage to Pulse Width Modulation (PWM) converter, and a power supply unit. The current transducer measures current during charge or discharge, and outputs a corresponding voltage signal. The voltage to PWM converter converts the voltage signal generated by the current transducer to a PWM signal. The power supply unit supplies power to the current transducer.
In one preferred embodiment, the current transducer is an open-loop Hall effect current sensor. Also, preferably, the voltage to PWM converter and power supply unit is formed as a hybrid Integrated Circuit (IC) chip, and the chip is installed inside the current transducer.
In another preferred embodiment of the present invention, the voltage to PWM converter includes a triangle wave generator, a voltage level detector, and an open-collector transistor circuit. The triangle wave generator generates a triangle wave. The voltage level detector generates a PWM signal by comparing the voltage signal generated by the current transducer to the triangle wave input from the triangle wave generator. The open collector transistor circuit transmits the PWM signal to other control units.
Preferably, the voltage to PWM converter further includes a voltage follower for preventing the voltage signal of the current transducer from being distorted by a change of impedance of the current transducer, and for transmitting the voltage signal generated by the current transducer to the voltage level detector.
In another preferred embodiment of the present invention, the power supply unit comprises a DC/DC converter for converting a power level to the level that is used in the current transducer, and for supplying the converted power to the current transducer.
In yet another preferred embodiment, a system for measuring a current of a battery for an electric vehicle includes a current level detector, an analog-to-digital waveform converter, and a power supply unit. The current level detector measures battery current during charging and discharging states of the battery, and generates a corresponding voltage signal, using substantially unipolar power. The analog-to-digital waveform converter converts the voltage signal generated by the current transducer to a substantially digital waveform, and the power supply unit supplies unipolar power to the current transducer.
Preferably, the current level detector is an open-loop Hall effect current sensor, and the analog-to-digital waveform converter is a voltage-to-PWM converter. Also, in another preferred embodiment, the system further includes an open-collector transistor circuit for transmitting the substantially digital waveform to other control units.